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OPA381AIDGKR Datasheet(PDF) 10 Page - Texas Instruments |
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OPA381AIDGKR Datasheet(HTML) 10 Page - Texas Instruments |
10 / 24 page OPA381 OPA2381 SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004 www.ti.com 10 BIASING PHOTODIODES IN SINGLE-SUPPLY CIRCUITS The +IN input can be biased with a positive DC voltage to offset the output voltage and allow the amplifier output to indicate a true zero photodiode measurement when the photodiode is not exposed to any light. It will also prevent the added delay that results from coming out of the negative rail. This bias voltage appears across the photodiode, providing a reverse bias for faster operation. An RC filter placed at this bias point will reduce noise. (Refer to Figure 4.) This bias voltage can also serve as an offset bias point for an ADC with range that does not include ground. OPA381 V OUT =IDRF +VBIAS 100k Ω V+ R F 10M Ω I D C F (1) < 1pF 0.1 µF λ NOTE: (1) C F is optional to prevent gain peaking. It includes the stray capacitance of R F. +V BIAS [0V to (V+) − 1.8V] Figure 4. Photodiode with Filtered Reverse Bias Voltage TRANSIMPEDANCE AMPLIFIER Wide bandwidth, low input bias current and low input voltage and current noise make the OPA381 an ideal wideband photodiode transimpedance amplifier. Low voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequency. The key elements to a transimpedance design are shown in Figure 5: D the total input capacitance (CTOT), consisting of the photodiode capacitance (CDIODE) plus the parasitic common-mode and differential-mode input capacitance (2.5pF + 1pF for the OPA381); D the desired transimpedance gain (RF); D the Gain Bandwidth Product (GBW) for the OPA381 (18MHz). With these three variables set, the feedback capacitor value (CF) can be set to control the frequency response. CSTRAY is the stray capacitance of RF, which is 0.2pF for a typical surface-mount resistor. To achieve a maximally flat 2nd-order Butterworth frequency response, the feedback pole should be set to: 1 2 pR F CF ) C STRAY + GBW 4 pR FCTOT Bandwidth is calculated by: f*3dB + GBW 2 pR FCTOT Hz These equations will result in maximum transimpedance bandwidth. For even higher transimpedance bandwidth, the high-speed CMOS OPA380 (90MHz GBW), the OPA300 (150MHz GBW), or the OPA656 (230MHz GBW) may be used. For additional information, refer to Application Bulletin AB−050 (SBOA055), Compensate Transimpedance Amplifiers Intuitively, available for download at www.ti.com. C TOT (3) OPA381 V OUT −5V 10M Ω +5V R F C F (1) C STRAY (2) λ NOTE: (1) C F is optional to prevent gain peaking. (2) C STRAY is the stray capacitance of RF (typically, 0.2pF for a surface−mount resistor). (3) C TOT is the photodiode capacitance plus OPA381 input capacitance. R P (optional pulldown resistor) Figure 5. Transimpedance Amplifier (1) (2) |
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